Research and standardization are both very active in defining new modulation formats and technologies for 100 G and 400 G short reach optical interconnection interfaces, having a reach of up to 2 km. Applications for short reach optical interconnections include intra data centre fibre links and connections to radio units for massive MIMO, multiple input multiple output, or beamforming. The design guidelines for short reach optical interconnection interfaces are very different from those that guide the design of optical transport line cards, with energy efficiency, low cost and low latency replacing spectral efficiency and distance as the main design criteria. Current interconnection solutions for 100 Gbit/s data rates include 4×25 GBit/s coarse wavelength division multiplexing, CWDM, 4×25 Gbit/s over parallel fibres, known as parallel single mode 4 lane, PSM4, pulse amplitude modulation, PAM-4, and discrete multi-tone, DMT.
These solutions provide only a partial answer to the demand for high interconnection speeds, high energy efficiency or a combination of both. The current technology limits the number of optical components that can be integrated in a single chip. For example, CWDM4 and PSM4 rely on four 25 Gbit/s transmitters to achieve an aggregate rate of 100 Gbit/s. To increase the bit rate further would require the use of a multi-level modulation format to overcome transmitter and receiver bandwidth limitations. However, multi-level modulation formats, like DMT and PAM-4, require the use of both digital signal process, DSP, which dissipates power, and forward error correction, FEC, which introduces latency.
More energy efficient, but less spectrally efficient, modulation formats, such as pulse position modulation, PPM, and frequency shift keying, FSK, exist and are used when the energy per transmitted bit needs to be minimized and the propagation medium does not introduce significant bandwidth limitation, such as in free-space communications. In principle, these modulation formats could also be used for interconnection links over dedicated fibre but so far their application has been limited by the high transmitter cost; for example, many lasers are needed for FSK generation.